At least 24 people were killed on Tuesday in a Daesh attack near a refugee camp on the Syrian side of the border with Iraq, a Britain-based war monitor said.
The Syrian Observatory for Human Rights said militants sneaked into the village of Rajm Sleibi, which houses the camp for the displaced people fleeing Daesh-held areas in Syria and Iraq. The village lies in Hasakeh province – a frontline that separates the area from Daesh-held places further south.
"At least five suicide attackers blew themselves up outside and inside a camp for Iraqi refugees and displaced Syrians in Hasakeh province," Observatory chief Rami Abdel Rahman said.
The dead included displaced children.
Heavy clashes then erupted between the Daesh fighters and members of the US-backed Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab fighters, some of whose combatants were among the dead, Abdel Rahman said.
The SDF is dominated by the YPG, which is the armed wing of the PYD, a Syrian affiliate of the PKK, listed as a terrorist organisation by Turkey, the US and EU.
Redur Khalil, one of the spokespersons within SDF, said the attack came a few hours after Daesh suicide bombers dressed in civilian clothes entered the town of Shaddadeh and engaged SDF forces, triggering clashes.
Daesh is under attack by an array of forces in Syria and Iraq.
In Syria, the SDF is now fighting to recapture the town of Tabqa from the group.
Tabqa is an important stronghold for the militants, located about 40 kilometres (25 miles) southeast of Daesh's de facto capital, the city of Raqqa.
In Iraq, the extremist group is fighting for survival against Iraqi forces and their allies in the last neighbourhoods it still holds in the western part of Mosul, Iraq's second largest city.
Human Rights Watch on Monday accused Syrian government forces of likely dropping bombs containing nerve agents at least three times elsewhere in the country before an April 4 attack that killed dozens of people and sparked a retaliatory US strike.
Bashar al Assad's forces are also stepping up chlorine gas attacks and have begun using surface-fired rockets filled with chlorine in fighting near Damascus, the US-based rights group said in a new report.
"The government's use of nerve agents is a deadly escalation – and part of a clear pattern," said Kenneth Roth, Human Rights Watch's executive director.
"In the last six months, the government has used warplanes, helicopters, and ground forces to deliver chlorine and sarin in Damascus, Hama, Idlib and Aleppo."
The Organisation for the Prohibition of Chemical weapons, a global watchdog, has said sarin or a similar banned toxin was used in the April 4 strike.
Human Rights Watch said that before the April 4 attack on Khan Shaykhun, government warplanes also appeared to have dropped nerve agents on eastern Hama on December 11 and 12, 2016, and northern Hama, near Khan Shaykhun, on March 30, 2017.
Three suspected attacks in Hama
Human Rights Watch said 64 people died from exposure to nerve agents after warplanes attacked territory controlled by Daesh in eastern Hama on December 11 and December 12.
Activists and local residents provided names of the victims, while Human Rights Watch interviewed four witnesses and two medical personnel about the alleged attacks.
A third suspected nerve agent attack in northern Hama on March 30 caused no deaths but injured dozens of civilians and combatants, according to residents and medical personnel, the report said.
The alleged attacks were systematic and in some cases directed against civilians, which would meet the legal criteria to be characterised as crimes against humanity, the rights group said.
Syrian regime denies accusations
The Syrian regime has repeatedly denied using chemical weapons. The Syrian UN mission was not immediately available to comment on the allegations by Human Rights Watch, which cited interviews with witnesses and medical personnel.
Last month, Assad told AFP in an interview that the suspected sarin attack in Khan Shaykhun was "100 percent" fabricated, serving as a pretext for US missile strikes on a Syrian airfield.